1. Field of the Invention
The present invention relates to a light source device and a laser scanning device used as an optical writing unit such as a laser beam printer (LBP), digital plain paper copier (PPC) or the like.
2. Description of the Related Art
Conventional image writing performed by laser beam printers and digital plain paper copiers is accomplished using a laser scanning device. In the light source device comprised of a laser light source and collimator lens for confining the laser light in parallel rays and which is among the structural components of this laser scanning device, the laser light source generates heat which causes thermal expansion of the members which retain heat, thereby causing fluctuation of the distance between the collimator lens and light generation point of the laser light source. When a semiconductor laser element is used as a laser light source, the wavelength of the output laser beam fluctuates in accordance with the temperature fluctuation. As a result, the convergence state of the laser beam emitted from the light source changes, and causes changes in the spot diameter of image formation of the laser beam on the scanned surface, thereby precluding the formation of fine detail images.
Means have been proposed to rectify the aforesaid disadvantage, such as, for example, Japanese Unexamined Patent Application No. HEI 4-320079, which discloses a light source comprising a laser support member which supports a laser light source, and a lens support member which supports a collimator lens connected to said lens support member. In this light source, expansion of the laser support member due to heat elevation, and fluctuation of the oscillation wavelength of the laser light source mutually cancel one another, such that the spot on the scanned surface does not change regardless of temperature fluctuation.
Another conventional construction is known wherein the change in the focal length induced by spatial change between the semiconductor laser element and the collimator lens is compensated for by a plastic lens. This solution utilizes the heat-induced change in the shape and refractive index of the plastic lens.
FIG. 23 briefly shows the construction of a light source device used in the aforesaid conventional laser scanning devices. This light source device 120 comprises a laser light source 121, collimator lens 122, support member 123, and lens barrel 124.
In general, when a semiconductor laser element is used as a laser light source 121, the emission position inescapable varies during manufacture. Accordingly, in order to maintain errors arising from discrepancies in the laser emission position which affect the focal length of the entire optical unit within a permissible range, the distance separating the laser light source 121 and collimator lens 122 must be individually adjusted during assembly.
Therefore, a light source device 120, which includes a support member 123 for stationary support of laser light source 121 and a lens barrel 124 for supporting a collimator lens 122, normally comprises at least two or more members. The support member 123 and lens barrel 124 have different required characteristics (e.g., hardness, processing characteristics, thermal conductivity and the like), and therefore are made of different materials.
In the conventional light source device 120 shown in FIG. 23, the support member 123 and the lens barrel 124 formed of different types of materials are joined at a surface 125 parallel to the optical axis 126 of collimator lens 122. In this construction, when laser light source 121 generates heat, the support member 123 and lens barrel 124 are subject to thermal expansion at respectively different linear thermal coefficients of expansion, such that the joining surface 125 between the support member 123 and lens barrel 124 is subject to both rubbing and smooth sliding. That is, support member 123 and lens barrel 124 may adversely affect reproducibility due to thermal variation.
When the positional relationship between support member 123 and lens barrel 124 is not displaced with excellent reproducibility in conjunction with temperature variation, the actual amount of change cannot be accurately predicted relative to temperature fluctuation regardless of a calculated value of change even when the change in the distance between the laser light source 121 and collimator lens 122 induced by temperature fluctuation is calculated using a linear thermal coefficient of expansion.
Accordingly, when a conventional laser light source device 120 is used as a light source of a laser scanning device, it cannot be predicted how the total focal length of the laser scanning device will be changed by temperature, such that the change in the total focal length of the optical unit induced by temperature fluctuation of said part is extremely difficult to correct by moving the other lenses in the optical unit or changing the properties of a specific lens.